Japanese Patent Application Laid-Open Publication No. 2003-69012 corresponding to U.S. Pat. No. 6,764,963 discloses a conventional method which performs treatment with hydrogen annealing or a wet atmosphere in a SiC semiconductor device using a crystal a-face with a (11-20) face orientation for a channel, thereby reducing an interface state density in a MOS structure and improving a channel mobility. Specifically, the reduction in interface state density is achieved by selecting a concentration or a temperature for the hydrogen annealing or the wet atmosphere.
FIGS. 22A and 22B are crystal-face schematic views showing the relationships between a hexagonal crystal structure of SiC and the surface orientations of crystal faces, i.e., a (11-20) face, a (0001) Si face, and a (000-1) C face. As shown in FIG. 22B, the upper and lower surfaces of the hexagonal crystal correspond to the (0001) Si face and the (000-1) C face, respectively, to which the (11-20) face is in perpendicular relation.
A face perpendicular to the (0001) Si face, such as the (11-20) face, is smaller in wafer diameter than the (0001) Si face due to problems associated with a crystal growth technique. Accordingly, it is desired to use a wafer with the (0001) Si face or the (000-1) C face. However, even when 5-hour wet oxidation at a high temperature of 1080° C. is performed with respect to the wafer with the (0001) Si face, only an oxidation film with a thickness of about 40 nm is formed thereon. Therefore, the wafer with the (0001) Si face has the problems that it cannot be thickened by thermal oxidation and that CMP polishing for processing the substrate surface using an oxidation reaction takes a time. On the other hand, the wafer with the (000-1) C face has an oxidation rate about 20 times higher than that of the wafer with the (0001) Si face. This allows easy formation of a thick thermal oxidation film and also allows high-speed CMP polishing. Therefore, the wafer with the (000-1) C face is more excellent in mass productivity than the wafer with the (0001) Si face, and a MOSFET using the wafer with the (000-1) C face is highly expected. However, a MOS interface formation method which reduces the interface state density at the (000-1) C face has not been disclosed. In Japanese Patent Application Laid-Open Publication No. 2003-69012 shown above also, only a method of reducing the interface state density at the (11-20) face is disclosed, and a technique for reducing the interface state density at the (000-1) C face is not disclosed.
Thus, it is required for a SiC semiconductor device to improve a channel mobility.